ng tides; sometimes they
act against one another, and we have neap tides. These effects are
further complicated by a number of other factors, and the tides, at
various places, vary enormously. Thus at St. Helena the sea rises and
falls about three feet, whereas in the Bay of Fundy it rises and falls
more than fifty feet. But here, again, the reasons are complicated.

Sec. 17

Origin of the Moon

But there is another aspect of the tides which is of vastly greater
interest and importance than the theory we have just been discussing. In
the hands of Sir George H. Darwin, the son of Charles Darwin, the tides
had been made to throw light on the evolution of our solar system. In
particular, they have illustrated the origin and development of the
system formed by our earth and moon. It is quite certain that, long ages
ago, the earth was rotating immensely faster than it is now, and that
the moon was so near as to be actually in contact with the earth. In
that remote age the moon was just on the point of separating from the
earth, of being thrown off by the earth. Earth and moon were once one
body, but the high rate of rotation caused this body to split up into
two pieces; one piece became the earth we now know, and the other became
the moon. Such is the conclusion to which we are led by an examination
of the tides. In the first place let us consider the energy produced by
the tides. We see evidences of this energy all round the word's
coastlines. Estuaries are scooped out, great rocks are gradually reduced
to rubble, innumerable tons of matter are continually being set in
movement. Whence is this energy derived? Energy, like matter, cannot be
created from nothing; what, then, is the source which makes this
colossal expenditure possible.

The Earth Slowing down

The answer is simple, but startling. _The source of tidal energy is the
rotation of the earth._ The massive bulk of the earth, turning every
twenty-four hours on its axis, is like a gigantic flywheel. In virtue of
its rotation it possesses an enormous store of energy. But even the
heaviest and swiftest flywheel, if it is doing work, or even if it is
only working against the friction of its bearings, cannot dispense
energy for ever. It must, gradually, slow down. There is no escape from
this reasoning. It is the rotation of the earth which supplies the
energy of the tides, and, as a consequence, the tides must be slowing
down the earth. The tides act as a kind